Missing Free Space on SYS

I have a NW65sp8 server which has a 20 GB SYS Volume and is showing approx 6 GB of Free Space when I look in NSSMU, NoRM, C1 and Windows Explorer.

I recently purged approx 9 GB from this Volume (in NoRM) but it is still showing only 6 GB of Free Space.
I have manually counted up the sizes of the folders in SYS to equal just under 5 GB.

There are no further files to purge (of any significance anyway).
There are no massive log files anywhere.
I have restarted the Server.
I have used the "NSS /POOLREBUILD=SYS /PURGE" (not just once).
There is no Compression on the Volume.
I've been through TIDs and the Forums but cannot find the answer.

Has anyone else come accross this type of thing ?
It isn't affecting the running of the server \ network but it is annoying me, please help.

Thanks
Neil